This invention relates generally to semiconductor technology and more particularly to a method of etching metal using fluorine-based chemistry.
As semiconductor technology continues to develop, a variety metals are being used in a wider variety of applications. Many of these metals present processing challenges, including problems associated with properly etching these metals. Platinum and iridium, for example, are finding use in ferroelectric memory devices and other devices. At the same time these metals are coming into wider use, the overall dimension of the devices continues to shrink. As the devices shrink, critical dimensions (CD) need to be maintained to greater accuracy.
Maintaining critical dimensions is made more difficult in the case of some metals, such as platinum and iridium, in that a suitable method of etching to pattern devices has not been available. Many of the present methods of etching, including plasma etching, produce residue that redeposits onto the substrate.
Plasma etching is accomplished by a combination of two mechanisms, physical etching (also referred to as xe2x80x9csputteringxe2x80x9d), and chemical etching. Prior art methods of etching platinum and iridium have involved primarily sputtering of the metal with some chemical etching. The chemical etching that has been reported involves the formation of non-volatile etch products. Sputtering is a form of physical etching that works by bombarding the surface with particles causing the underlying material to be removed by the force of the impact.
Prior art methods of platinum etching with argon/halogen mixed gas plasmas, and similar methods, utilize predominately sputtering, which produces a redeposition of etch products along mask sidewalls. Prior art methods also utilize chemical reactions in part, but produce etch products that are non-volatile. These non-volatile etch products are often redeposited as well. For example, prior art methods often use chlorofluorocarbons or other chlorine containing gases. Iridium, platinum and other metals form metal-chlorides that are non-volatile at reasonably obtainable process temperatures and conditions. This redeposition of sputtered material and non-volatile etch products produce structures commonly referred to as fences, because the redeposited material remains even after the mask has been removed. Several methods exist for removing the fences, however the overall process does not maintain critical dimensions well. In general, it is not uncommon for the etching of patterned platinum structures to produce feature sizes that consistently exceed the original dimensions of the mask. This increase in dimension has also been noted for so called xe2x80x9cfence-freexe2x80x9d platinum structures. Further investigation has revealed that so called xe2x80x9cfence-freexe2x80x9d patterned platinum structures, are often achieved through processes that produce a transient fence and then through continued etching remove the transient fence. This results in a change in both sidewall angle, and overall dimensions.
Another problem associated with prior art etch techniques is redeposition of etch products generally. The redeposition, which produces fence structures, can also cause other sorts of failures including possible electrical shorts within the final device due the metal content of the redeposited material. The prior art solution to this problem has been to add additional cleaning steps of various kinds, including wet etch, plasma ash, and standard wet clean processes. These additional processes can also add complexity and uncertainty to the processes, resulting in less overall control of critical dimensions.
Previous methods used a quartz tube reaction chamber at pressures above 100 mTorr. This configuration did not allow for direction control of the reactants. Also, at higher pressures within those chambers chemical etching was isotropic (meaning that etching occurs in multiple directions). Isotropic etching causes undercuts, which lead to greater difficulty in forming desired structures and in controlling critical dimensions.
Current methods of etching platinum and iridium generally have etch rates of less than 400 angstroms per minute.
It would be advantageous to have a method of etching platinum, iridium and certain other metals that would not produce fence structures.
It would be advantageous to have a method of etching platinum, iridium and certain other metals that would allow for greater control of critical dimensions.
It would be advantageous to have a method of etching platinum, iridium and certain other metals that reduces, or eliminates, the amount of redeposited residue.
It would be advantageous to have a method of etching platinum, iridium and certain other metals that was anisotropic.
It would also be advantageous to have a method of etching platinum, iridium and certain other metals at a rate faster than 400 angstroms per minute.
Accordingly, a method of etching, or removing, metal from selected areas of a substrate is provided. First a substrate is prepared for further processing according the present method, with any desired circuit structures previously formed on the substrate. Then metal is deposited over the substrate. The present method is well suited for use with metals that form volatile compounds with fluorine, such as, iridium, platinum, ruthenium, osmium, and rhenium. A substance is generally considered volatile if it readily vaporizes at a relatively low temperature. As used herein, the term xe2x80x9cvolatilexe2x80x9d characterizes substances that readily vaporize at temperatures and pressures readily obtainable in connection with the processing of semiconductor devices. Preferred ranges of temperature and pressure are provided within the detailed description.
Once the metal has been deposited, a mask layer is formed and patterned to expose selected areas of the metal. Methods of forming and patterning masks are well known. One common type of mask is a photoresist mask. Another common type of mask is referred to as a hard mask, which is commonly formed using silicon dioxide or silicon nitride. Photoresist masks are suitable for use of the present method at lower temperatures, but hard masks are generally preferred at higher temperatures, which may be desirable when practicing the present method. Photoresist has a tendency to become unreliable, or breakdown, at lower temperatures than hard mask materials.
After patterning, the metal is heated and exposed to a fluorine-containing plasma within a chamber. The chamber is preferably an ECR plasma etch chamber at a pressure of between 5 and 50 mTorr. The metal is heated by heating the substrate which is placed on a heated chuck. By controlling the temperature of the chuck, it is possible to control the temperature of the substrate and metal. The metal is heated to a point where it will produce a volatile metal-fluoride compounds when exposed to the fluorine within the plasma, typically the temperature is above 150 degrees Celsius for iridium and above 190 degree Celsius for platinum. The volatile metal-fluoride compounds essentially forms a vapor within the chamber. The vapor is then exhausted by a pump.
By producing and volatile compounds, the present invention reduces, or eliminates, residue. Due to the additional effects of sputtering within the chamber as well as the presence of other gases or materials which may form non-volatile compounds, some intermediate residue may be inadvertently deposited. Preferably, the amount of residue that is redeposited will be slight, and continuing etch effects during this process will eliminate residue.
Since chlorine forms non-volatile compounds with for example iridium and platinum, as well as other metals, it is preferable to use a source of fluorine that does not include chlorine. Therefore, chlorofluorocarbon gases will preferably not be used as the source of fluorine. The preferred sources of fluorine are carbon tetrafluoride (CF4), nitrogen trifluoride (NF3) or sulfur hexafluoride (SF6). Oxygen may also be introduced into the chamber to free up additional fluorine. In addition, inert dilutant gases, such as, argon, neon or helium or other common process gases may be introduced into the chamber.